The invention relates to expansion chambers or exhaust mufflers for two-stroke engines but may be utilized with any internal combustion engine having a phase wherein the intake and outlet ports are simultaneously open for a brief interval. Expansion chambers are a preferred means for achieving higher performance from motor vehicles such as motorcycles and replace stock mufflers. A good discussion of expansion chambers is contained in a booklet entitled "Two-Stroke Tuner's Handbook" by Gordon Jennings (1973). As discussed in that booklet on page 51, expansion chambers are an effective device for improving engine performance but have the problems of bulk, noise and inflexibility of mounting position. However, as Mr. Jennings states "unfortunately, . . . there is nothing else in an engineer's bag of tricks that comes anywhere close to matching the boost a two-stroke engine gets from a properly designed expansion chamber exhaust system."
The present invention obviates the bulk and the noise of conventional expansion chambers by in effect folding the chamber in an analogy to a spiral such that its length is compressed. The invention further provides for variable tuning by means of a removable plate which can modify the nominal length of the chamber to "fit" the particular engine porting and the desired optimum RPM of the engine. The length is important because, as is generally known, the distance from the exhaust port at which a sound reflective surface is placed to reflect a positive sound wave to the exhaust port timed to bar the emergence of the fuel charge into the exhaust system before the engine piston closes the exhaust port is critical. This distance is expressed in a formula which is emperical in which port timing in degrees times the velocity of sound in feet per second is divided by the desired revolutions per minute. The invention uses the data derived from this formulation for the basic length of the chamber, compressed in accordance with the invention, and then modifies the effective length in accordance with performance demands by changing the width of the inner divider plate.